The mainstream magnets used in color picture tubes to make electron beam adjustment are bonded magnets (resin-bonded magnets) comprising a magnetic powder, i.e., barium ferrite in general, which is bonded with a resin material. These bonded magnets suffer, however, from the problem that since the temperature coefficient is as large as about 0.2%/.degree.C., the bonded magnets expand as the temperature rises, causing a reduction in the magnetization, so that undesirable color shift may occur. In color display tubes, the resolution is lowered by misconvergence due to the reduction in the magnetization. This is a fatal disadvantage to picture tubes having requirements for high definition. To solve this problem, it is conventional practice to use a bonded magnet material comprising a magnetic powder of low temperature coefficient, e.g., alnico magnetic powder, to form an electron beam adjusting device.
Incidentally, a minute observation of the action of two-pole magnets and that of four- and six-pole magnets reveals that the amount of beam shift made by the two-pole magnets is several times that by the four- or six-pole magnets. The reason for this is as follows: The two-pole magnets are used to make a color beam from each electron gun coincident with the axis of the picture tube, and it is also necessary to consider variations in production of picture tubes and the effect of the earth magnetism. Therefore, the amount of beam shift in the two-pole magnets is set to about 9 mm in radius. In contrast, for the four- and six-pole magnets, since these are used to converge the color beams in the center of the picture tube and the convergence of the color beams is mostly determined by the design of the electron guns, the amount of beam shift is set to about 3 mm in radius.
In view of the fact that the magnitude of beam shift is proportional to the magnetization of the magnet (i.e., the beam shift is about 1 mm to a magnetization of 1 G), it will be understood that it is sufficient and preferable to set the magnetization of the four- and six-pole magnets to a value lower than that of the two-pole magnets.
In regard to the magnetization characteristics of alnico magnetic powder, since the coercive force of this material is smaller than that of the conventional barium ferrite magnetic powder, it is difficult to control the level of magnetization for each pair of magnets. In particular, in the case of magnetization at low level, variations in the amount of magnetization are likely to occur, raising difficulty in making convergence adjustment; in the worst case, fatal problems arise, including a failure to effect the convergence adjustment.
In addition, since alnico magnetic powder is considerably costly as compared with barium ferrite magnetic powder, an increase in the alnico magnetic powder content leads to a rise in the production cost.
In view of the above-described circumstances, it is an object of the present invention to provide an electron beam adjusting device in which each pair of constituent magnets can be readily magnetized with minimal magnetization variations and which is less costly.